We consider the shift transformation on the space of infinite sequences over a finite alphabet endowed with the invariant product measure, and examine the presence of a hole on the space. The holes we study are specified by the sequences that do not contain a given finite word as initial sub-string. The measure of the set of sequences that do not fall into the hole in the first $ n $ iterates of the shift is known to decay exponentially with $ n $, and its exponential rate is called escape rate. In this paper we provide a complete characterization of the holes with maximal escape rate. In particular we show that, contrary to the case of equiprobable symbols, ordering the holes by their escape rate corresponds to neither the order by their measure nor by the length of the shortest periodic orbit they contain. Finally, we adapt our technique to the case of shifts endowed with Markov measures, where preliminary results show that a more intricate situation is to be expected.
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Figure 2. The relative error between a very precise numerical approximation of $ \gamma^r_{max} $ and the lower bound $ l_b $ in Corollary 3.9, defined by $ RE(r): = (\gamma^r_{max} - l_b)/ \gamma^r_{max} $ and displayed as a function of the length $ r $ of the hole in log-linear scale. The decay towards zero shows that the accuracy of the estimate improves exponentially with the length of the hole. Different curves correspond to different values of $ p $ (from bottom to top: $ p = 0.85 $, $ p = 0.9 $, $ p = 0.95 $)
Figure 3. The escape rates for the system of Section 4, as functions of $ \pi_{aa},\pi_{bb}\in (0,1) $: the blue graph is for the holes $ w = (aaa) $ and $ w = (bbb) $; the red graph is for the holes $ w = (aab) $, $ w = (bba) $, $ w = (baa) $ and $ w = (abb) $; the green graph is for the holes $ w = (aba) $ and $ w = (bab) $
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The escape rates
The relative error between a very precise numerical approximation of
The escape rates for the system of Section 4, as functions of